Montero (0-3) was called up from Triple-A Las Vegas earlier in the day after impressive rookie Jacob deGrom went on the disabled list with rotator cuff tendinitis.

Montero struck out two in the first inning before the Nationals began to tee off. Mets manager Terry Collins said the home runs were “very concerning.”

“You have to keep the ball in the ballpark,” Collins said. “I know he’s not afraid to throw it over the plate, but he needs to make better pitches.”

The 23-year-old righty made his big league debut this year and started four times before getting sent down after pitching on May 30. He was the Pacific Coast League pitcher of the week before this promotion.

Montero has allowed four home runs in 80 minor league innings this season. In the majors, he has yielded eight shots in just 25 innings.

“The batters here are a little bit better,” he said.

Montero was pulled after Desmond’s two-run homer in the sixth.

“Well, it doesn’t feel great but it doesn’t discourage me. It’s just part of the game. It’s going to happen every once in a while and as a pitcher I just have to keep working through that,” Montero said.

Collins said Montero would stay in the rotation.

“I’m going to talk to him, but you see he’s got the weapons that he can be successful here with,” he said. “But he’s got to learn to locate them better.”

After Rendon and Desmond homered in the sixth, Michael Taylor connected off reliever Carlos Torres later in the inning. The NL East leaders won their ninth straight at Citi Field.

Elevated from Triple-A over the weekend, Taylor singled in his first big league at-bat and later hit a two-run drive the opposite way at one of the majors’ most spacious parks.

A ball boy ran out to right field to pick up Taylor’s home run ball, but mistakenly tossed it to a young fan in the stands. Alerted to his error, the ball boy and a couple of security guards quickly worked out a trade of another ball to reclaim the prize.

Taylor finished 2 for 4. The 23-year-old outfielder hit a combined .315 with 22 homers and 35 stolen bases in the minors this season, almost all of it at Double-A.

Doug Fister (12-3) came back from a 25-minute rain delay and pitched seven shutout innings. He gave up seven hits, walked none and struck out three. Fister also hit a single for good measure after a 1-for-30 start at the plate this year.

Asdrubal Cabrera added two doubles and a single as the Nationals improved to 23-4 at Citi Field dating to 2011. The only Washington starter who didn’t get on base was leadoff man Denard Span, ending his 14-game hitting streak and his string of reaching safely at 36 games.

Mets RHP Matt Harvey (Tommy John surgery) might not pitch in any games until spring training. Harvey kept saying he wanted to throw in the majors this year while the Mets thought outings in the Arizona Fall League or instructional ball were more realistic. But general manager Sandy Alderson says Harvey and the team will now talk about cutting back on Harvey’s work and limit him to bullpen sessions. Alderson was concerned that RHP Jeremy Hefner recently had a setback in his recovery from Tommy John surgery and might need more reconstructive work. “My sense is that Matt will at least take this into account, as we will,” Alderson said.

Over the last nine matchups on the Mets’ field, Washington has outscored them 67-18. … This was the fifth time that the Nationals have hit at least four home runs in a game at Citi Field, all in the last two seasons. The Mets have had a total of five games with at least four homers at Citi Field since the park opened in 2009.